The use of electrophoretic primers has greatly increased in the automotive industry due to the superior coating obtained in many applications. This type of primer provides a highly uniform film thickness and is capable of reaching highly inaccessible areas which are difficult to coat by other painting processes. This increased usage, however, has also seen a rise in surface pitting or cratering in the electrophoretic primer coating which detracts from surface finish and often requires refinishing of the primer coating before application of a color coat.
In examining this cratering problem it has been determined that cratering principally occurs from four entirely different sources: (a) oil deposited on the phosphated steel prior to reaching the electrocoat tank; (b) oil floating on the top of an electrocoat bath; (c) oil emulsified into the electrocoat bath; and (d) oil on the coated surface of an uncured electrocoat film. The last source has been found to be a major source of crater problems and investigation has shown that oils and various metal working lubricants are frequently trapped in flanges and other tight fitting areas. These oils and lubricants were found to be expelled onto the uncured electrophoretic primer during the baking operation used to cure the primer causing craters to form.
Much attention is being given to cleaning and washing procedures to insure removal of such oil and lubricants before the metal is washed with primer but complete removal appears unobtainable as some material is frequently trapped in hem-flanges, etc. Accordingly, the metal working chemical art has turned attention to developing oils and metal working materials which have greater compatibility with uncured electrophoretic primers. Due to the many variations in metal and primer compositions, however, it was determined that a test procedure to identify the mill oils and drawing compounds which cause cratering in any given application would be highly desirable.